Die is used by turtles and links. When a turtle or link is asked to die, it no longer exists in the model, will not execute any more code, and will disappear from any agentset it was in. For example, if count turtles returned 10, then ask one-of turtles [die] was called, now count turtles would return 9.
Once you mastered the die primitive, don't stop there. Check out the resources below to improve your NetLogo skills.
die primitive:create-turtlescreates turtles with random colors and headings in the default shape of a turtle
create-links-withcreates links with every agent in an agentset